Many natural resource industry activities involve logging a well bore which penetrates an underground formation in a zone of commercial interest. The objective of the logging is to determine the in-situ properties and commercial value of one or more underground layers of special interest in this zone. These layers are typically sandwiched between thicker shoulder beds immediately over- and under-lying the layer of interest (i.e., the thick surrounding beds are not of commercial interest).
Thinner layers (i.e., generally less than 3 meters thick) are increasingly of commercial interest. For certain resources of increasing scarcity, such as oil and gas, the production potential from thin layers is now commercially viable. Accurate knowledge of the layer's resistivity may be critical to assessing the thin layer's commercial value.
Existing thin layer resistivity logging instruments are generally wire line supported. The wire line supplies electrical power and transmits signals from the instrument traversing the zone. The instrument contains either (1) electrical potential/current sources and electrical monitoring contact points (i.e., electrode devices), or (2) magnetic or other electromagnetic field sources and receivers (i.e., induction devices). Electrode devices can only be used where direct electrical contact to the formation materials can be maintained, e.g., using conductive drilling muds or metal liners. Induction devices, devices however, can be used in many non conductive as well as conductive well bore environments.
Some wire line induction instruments have two or more electromagnetic receivers located at different distances from a source. Instruments may also include focusing coils to shape the field generated by one or more sources. The multiple receivers produce several sets of data (or log charts) as the device moves, traversing the well bore in the zone of interest, e.g., a Dual Induction log. The dual log instruments are configured to obtain information from different radial depths (e.g., Deep and Medium depth logs) into the formation from the well bore. Still other logging devices or receivers may be used to read "near-depth" or "flushed" resistivity data, e.g., near-well bore alteration or mud cake data.
Existing induction instruments experience at least three perturbing influences. These are: (1) a conductive (or otherwise electromagnetic field interactive) influence from a fluids present in the well bore; (2) shoulder bed(s) within the extended responses of the induction device while it traverses near the thin bed; and (3) fluid alteration of the formation immediately surrounding the well bore.
The primary objectives of a layer resistivity logging method are to: (1) accurately estimate the layer's true (representative of in-situ) electromagnetic resistive properties; (2) tolerate various well directions, layer non-linearities, fluid property variations, near well bore fluid invasion and well bore alterations; and (3) permit logging of thin as well as thick layers at various (dip) angles to the well bore. The method should also be reliable, require little or no independent sources of input data and be simple to use.
Existing well bore resistivity analysis methods may do some of these objectives well. However, other objectives may be accomplished poorly or not at all. Existing dual log analysis methods begin with log data which generally represents resistivity under nominal or ideal conditions. The ideal resistivity estimate assumes a nominal well bore diameter, zero dip angle, non-conductive well bore fluid, infinitely thick layer or bed (i.e., no edge effects), and non-invaded formations. These methods then provide resistivity corrections or factors for actual well bore/instrument geometries, fluid properties, contamination/invasion effects, and layer/edge geometries. Corrections for thin layer geometries require an accurate, sometimes separate and independent source of data to estimate layer thickness.
The current thin layer corrections are essentially based upon extrapolations from ideal (thick) layer conditions and a measurement of actual layer thickness. As less of the (thin) layer and more of the shoulder beds come within the electromagnetic field sensed by the logging resistivity instrument, the accuracy of these extrapolations is limited or even non-existent. Accuracy of correction is especially sensitive to layer thickness data when shoulder bed resistivities are significantly different from the layer resistivity, which is common in oil and gas exploration. Extreme thin layer sensitivity (rapidly changing correction factor versus measured thickness) may require extrapolations of uncertain accuracy for thicknesses as large as approximately 2 meters (6 feet). Even these uncertain extrapolations may end for a measured layer than in the order of 1 meter (3 feet), typically 0.6 to 1.2 meters (2 to 4 feet).
In addition, accurate thin layer thickness measurements and data are especially difficult to obtain in dipping layers (i.e., layers which do not project perpendicularly from the well bore). This combination of factors makes existing resistivity determinations in thin layers dubious at best.
Efforts to improve the determination of resistivity of a dipping thin layer are known. An example of a theoretical approach is found in work done by R. H. Hardman and L. C. Shen, entitled "Theory of Induction Sonde in Dipping Beds." Although the analytical approach is theoretically retically accurate (if the assumptions used are correct), a measured value for "apparent" thickness of the layer from the induction log is needed. The "apparent" thickness is derived from the logged distance between the (theoretically) distinct changes of the resistivity (or other logging) data at the shoulder bed interfaces.
However, the distinct changes may not be present in actual logging data (e.g., the layer-shoulder bed transition occurs over a finite distance) or may be obscured by other factors. In addition, the resolution (typically measured in inches) of this type of logged data may be adequate for thick layers (typically measured in tens of feet) but can introduce serious errors in sensitive thin layer calculations.
These layer thickness limitations can be observed in the charts of correction factors developed by Hardman and Shen. These charts show resistivity being very sensitive to "apparent" bed thickness for thin layers, Still further, some of these charts only extend down to thicknesses of approximately 0.6 meter (2 feet). These theoretical and practical aspects make the measurement of "apparent" thickness a poor basis for accurate determination of resistivity of a thin layer of interest.
None of the current "true" thin layer resistivity analysis methods known to the inventor eliminates the problems of (1) requiring very accurate knowledge of "apparent" or actual thin bed thickness or (2) using uncertain or extrapolated correction factor information even if an accurate estimate of layer thickness is known. The invention provides a method to solve these two problems, as well as providing other advantages as will become clear in light of the following description.